Conventionally, the braking of a rail vehicle is performed in that compressed air is admitted to a brake cylinder, wherein a piston moves axially and transmits an axial brake force. As an alternative, most often used for parking and emergency braking but occasionally also for service braking, a powerful spring is normally held compressed by compressed air in a cylinder, but when the air pressure is lowered a brake force is exerted.
There is currently a trend towards avoiding a compressed air system on modern rail vehicles, which means that no air for control or power generation is available. In contrast it is often desirable to utilize electricity both as the power generating medium and the control medium, partly in view of the frequent use of electronics in control systems and the simplicity in the equipment for transferring power in the form of electricity, which can be used for diverse applications on board a modern rail vehicle.
Accordingly, it is a growing interest for the concept called "braking by wire", i.e. a system in which electric power is transformed into a mechanical brake force in relation to an electric signal supplied from the driver. The requirements on such a system are high, for example with regard to accuracy and response times in view of possible anti-skid functions and so forth, but also with regard to simplicity, reliability and ability to withstand the rather extreme environmental stresses underneath a rail vehicle.
Several attempts to accomplish designs fulfilling the different requirements on so called electro-mechanical brake units are known. Examples of solutions where an electric motor is used to tension a normal spring (a helical spring), which applies the brake force when desired, are disclosed in US-A-874 219, US-A-2 218 605, US-A-4 033 435, US-A-4 202 430, DE-A-3 010 335, GB-A-2 141 500, and EP-A-166 156.
There are also examples of solutions where the energy from the electric motor is stored in a coil spring or clock spring, namely US-A-3 131 788, US-A-3 217 843, and US-A-3 280 944. In these solutions, stemming from one source, the application of the brake is controlled by the motor, which also is used for tensioning the spring. By this technique it is virtually impossible to obtain the response times and control necessary in modern systems.